Within various fields of application regarding modern energy technologies efficient possibilities for storage of thermal energy is required.
One technique of storing thermal energy is achieved by providing a fluid, such as e.g. water, in a subterranean chamber. The thermal energy of the fluid is to a great extent conserved within the chamber during an extended period of time. Today this technique is used in different parts of the world so as to satisfy needs of storing thermal energy between different seasons.
Storage of thermal energy in caverns is mainly achieved by a temperature difference within water filled in an excavated cavern. As water has approximately double heat capacity compared to a corresponding volume of rock, efficient heat storage is achieved according to this method. The rock surrounding the cavern is used as insulation. Due to large scale effects a relatively good insulation is achieved. The surrounding rock also to some extent contributes to the storage function.
Due to high initial costs relating to preparation of caverns for storing thermal energy, few projects have been realized so far. One thermal energy storing plant is located in Lyckebo, Sweden. The plant for thermal energy storage in Lyckebo basically comprises a tunnel surrounding and an excavated cavern having storage volume of about 100.000 m3. Both the tunnel and the chamber are filled with water. The tunnel and the chamber define separated volumes for holding water for storage of thermal energy. The water contained in the tunnel and the chamber, respectively, are substantially in fluid communication with each other. However, as depicted below, the plant suffers from some drawbacks.
This particular plant has been in use since 1984 and is today still in operation. In the Lyckebo plant telescopic extraction pipes have been used. The telescopic extraction pipes are arranged to extract water at level in the cavern having a desired temperature and to return used water having another temperature at an appropriate level in the cavern. The warmest water in the storage may be spared until required. Use of the telescopic extraction and return pipes allows a distinct separation layer between warmer and colder water within the cavern to during use continuously be reduced to a minimum.
In the Lyckebo plant some unexpected leakage of water from the storage cavern to the surrounding access tunnel has been detected. Said leakage has resulted in an increased heat loss from the stored water by approximately 30%. The surrounding tunnel achieved a higher temperature than expected. Thermal energy stored in the tunnel has not been efficiently utilized in the plant.
The “surrounding access tunnel” in Lyckebo was initially designed to minimize required area for the storage. Another purpose was to equalize the ground water pressure around the cavern. A ground water flow would then enter into the tunnel “upstream” and leak out “downstream” from the tunnel. No further attempts were performed to minimize heat leakage due to ground water flow, if any.
It therefore exist a need to provide an improved arrangement for storing thermal energy under ground.